LLVM 19 and enabling `reference-types` by default

In LLVM 19 the reference-types feature is being enabled by default in https://github.com/llvm/llvm-project/pull/96584. This is having consequences in Rust - https://github.com/rust-lang/rust/issues/128475 - and is requiring documentation to be written - https://github.com/rust-lang/rust/pull/128511 - for how to disable features (docs that should be written anyway, but this update is becoming a strong forcing function).

The main consequence of enabling reference-types is that the table index immediate in the call_indirect instruction is now being encoded as an overlong uleb which is 5 bytes long as 80 80 80 80 00. This overlong encoding of 0 has no semantic difference from when reference-types were disabled but validators and parsers which don't support reference types are rejecting these modules.

I wanted to raise this issue here for awareness to confirm that this is expected fallout. I understand that enabling refernece-types by default is expected, but I wanted to additionally confirm that the consequences of actually emitting a breaking change into all modules using call_indirect relative to parsers that don't support reference-types is expected. This seems like surprising behavior to me where modules that don't use reference-types at all are now required to be executed in runtimes that support the reference-types proposal.

Or, alternatively, if there's an easy-ish way to shrink the leb encoding here (e.g. by assuming there's <= 127 tables and continuing to use a single byte) I think that'd personally be best to generate MVP modules as much as possible and only emit newer features when explicitly requested.

cc @sbc100, @aheejin, @tlively

Start by reading the linked LLVM pull request, Rust issue, and Rust documentation pull request, then examine the described call_indirect table-index encoding and reference-types behavior. This issue is complete only when the expected compatibility behavior is confirmed and documented, or a concrete encoding change is agreed upon.